Resource record always rendered as text/html

[kngenie]

Resource record is always rendered as text/html, regardless of Content-Type WARC header field.

This is due to a lack of metadata record support in JWATResource. It does not return Content-Type header field from its getHttpHeaders() method, thus Tomcat supplies default value text/html.

Known internally as WWM-126.

[kngenie]

It turned out JWATResource does not a test suite, and it is not fully compliant to Resource interface contracts. Implemented tests, and updated JWATResource to pass all tests.

Fix is in eea650b .. c52d8b3 and 69a4f08. Merged to master c1dd221.
Merged to iipc/master in 1dbd4f3 (iipc#195).